Experimental observation of the crystallization of a paired holon state

TL;DR

This study observes a charge excitation at 201 meV in a doped cuprate, indicating local pairing of holes that persists above the charge density wave transition, revealing insights into paired holon states in ladder cuprates.

Contribution

The paper provides experimental evidence of paired holon states in ladder cuprates through ultraviolet resonance Raman scattering, highlighting local pairing above the charge density wave transition temperature.

Findings

Charge excitation at 201 meV is of charge nature.

The excitation persists above the charge density wave transition temperature.

Doped holes form paired holons that order below T_{CDW}.

Abstract

A new excitation is observed at 201 meV in the doped-hole ladder cuprate Sr$_{14}$Cu$_{24}$O$_{41}$, using ultraviolet resonance Raman scattering with incident light at 3.7 eV polarized along the direction of the rungs. The excitation is found to be of charge nature, with a temperature independent excitation energy, and can be understood via an intra-ladder pair-breaking process. The intensity tracks closely the order parameter of the charge density wave in the ladder (CDW$_L$), but persists above the CDW$_L$ transition temperature ($T_{CDW_L}$), indicating a strong local pairing above $T_{CDW_L}$. The 201 meV excitation vanishes in La$_{6}$Ca$_{8}$Cu$_{24}$O$_{41+\delta}$, and La$_{5}$Ca$_{9}$Cu$_{24}$O$_{41}$ which are samples with no holes in the ladders. Our results suggest that the doped holes in the ladder are composite bosons consisting of paired holons that order below $T_{CDW}$.